1. Field of the Invention
This invention relates to a projection objective for short wavelengths, preferably of ≦193 nm, especially ≦157 nm, and also to a projection exposure apparatus and a chip production, each of which employs such a projection objective.
2. Description of the Related Art
Lithography with wavelengths of <193 nm is discussed as a possible technique for imaging structures of <130 nm and with special preference <100 nm. The resolution of a lithographic system is described by the following equation:RES=k1·(λ/NA)wherein k1 is a specific parameter of the lithographic process, λ is the wavelength of the incident light, and NA is the numerical aperture of the system on the image side.
For imaging systems in the wavelength region of <193 nm, essentially reflective components are available in addition to refractive components made of fluorides.
Purely reflective systems with six mirrors for microlithography with wavelengths of <193 nm have been disclosed by the publications U.S. Pat. No. 5,686,728 and EP 0 779,528, and U.S. Pat. No. 5,815,310.
The projection lithography system according to U.S. Pat. No. 5,686,728 shows a projection objective with six mirrors, with each of the reflective mirror surfaces being of aspherical design. The mirrors are arranged along a common optical axis in such a way that an obstruction-free light path is achieved.
To reduce the residual errors of the objective and to achieve larger apertures at the outlet pupil, EP-A-0 779,528 suggests introducing three refractive optical elements into the beam path. A drawback of this embodiment with few lenses is, that the image-side numerical aperture NA is equal to 0.6, and the refractive optical elements have large diameters, which is undesirable with respect to availability of materials and costs, as well as manufacturing feasibility. It is another drawback that the refractive optical elements are off-axis segments, which are difficult to mount and to align.
An objective that is similar to that from EP-A-0 778,528 is described in U.S. Pat. No. 4,701,035. FIG. 12 of U.S. Pat. No. 4,701,035, for example, shows an objective with nine mirrors, two lenses, and two intermediate images. The objective that is described in U.S. Pat. No. 4,701,035 has essentially the same drawbacks as the system known from EP-A-0 779,528, i.e. a small image-side numerical aperture. Furthermore, the optical elements, as in EP-A-0 778,528, are difficult to mount and to align.
Catadioptric projection objectives have been disclosed by EP-A-1,069,448 and WO-A-01/51979 with an image-side numerical aperture of NA >0.6. The objectives disclosed by both EP-A-1,069,448 and WO-A-01/51979 are systems centered around an optical axis, with the number of refractive elements always being greater than that of reflective elements. A drawback to these systems is their use of much lens material.